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Abstract

A facile solvothermal method to synthesize aluminum-doped ceria-zirconia (Ce0.5Zr0.5-xAlxO2-x/2, x = 0.1 to 0.4) solid solutions was carried out using Ce(NH4)2(NO3)6, Zr(NO3)3·2H2O Al(NO3)3·9H2O, and NH4OH as the starting materials at 200°C for 24 h. The obtained solid solutions from
the solvothermal reaction were calcined at 1,000°C for 20 h in air atmosphere to evaluate
the thermal stability. The synthesized Ce0.5Zr0.3Al0.2O1.9 particle was characterized for the oxygen storage capacity (OSC) in automotive catalysis.
For the characterization, X-ray diffraction, transmission electron microscopy, and
the Brunauer-Emmet-Teller (BET) technique were employed. The OSC values of all samples
were measured at 600°C using thermogravimetric-differential thermal analysis. Ce0.5Zr0.3Al0.2O1.9 solid solutions calcined at 1,000°C for 20 h with a BET surface area of 18 m2 g−1 exhibited a considerably high OSC of 427 μmol-O g−1 and good OSC performance stability. The same synthesis route was employed for the
preparation of the CeO2 and Ce0.5Zr0.5O2. The incorporation of aluminum ion in the lattice of ceria-based catalyst greatly
enhanced the thermal stability and OSC.

Keywords:

Background

Ceria (CeO2)-based materials have attracted considerable interest for more than half a century
due to their far-ranging applications in catalysts, fuel cells, cosmetics, gas sensors,
and solid-state electrolytes and especially their crucial application as promoters
of three-way catalysts (TWCs), which are commonly used to reduce the emissions of
CO, NOx, and hydrocarbons from automobile exhausts, because of their excellent oxygen storage
capacity (OSC) [1-8]. Since 1990s, CeO2-ZrO2 solid solutions have gradually replaced pure CeO2 as OSC materials in the TWCs to reduce the emission of toxic pollutants (CO, NOx, hydrocarbons, etc.) from automobile exhaust and because of their enhanced OSC performance
and improved thermal stability at elevated temperatures [9-13].

The redox property of CeO2 can be greatly enhanced by the incorporation of zirconium ions (Zr4+) into the lattice to form a solid solution [14-16]. Nagai et al. have suggested that enhancing the homogeneity of Ce and Zr atoms in
the CeO2-ZrO2 solid solution can improve the OSC performance [17]. The detailed structure and property of CeO2-ZrO2 solid solutions were reported in a review article by Monte and Kaspar [12]. This review included the results of reducing performance for a series of samples
with gradually elevated Ce contents, and a possible mechanism of structural changes
in the reducing process was proposed. Fornasiero et al. have reported that an optimum
composition like Ce0.5Zr0.5O2 (molar ratio of Ce:Zr = 1:1) can exist as a cubic phase, which can have a considerably
high redox property [18]. Using density functional theory, Wang et al. found that in a series of Ce1-xZrxO2 solutions with a content of 50%, ZrO2 possesses the lowest formation energy of the O vacancy; therefore, Ce0.5Zr0.5O2 exhibits the best OSC performance [19]. Recently, many researchers have paid much attention to prepare the Ce0.5Zr0.5O2 solutions with the homogeneity of the composition, good dispersion of particles,
narrow particle size distribution, better crystallinity, and high surface area in
order to improve OSC and redox property due to their catalytic applications [20-25].

Although Ce0.5Zr0.5O2 solid solutions have been studied extensively, there are few reports on the preparation
of Ce0.5Zr0.5-xMxO2-x/2 in the literature [26,27]. Considering the smaller cation radius of Al3+ (0.059 nm) compared to those of Zr4+ (0.084 nm) and Ce4+ (0.097 nm), the incorporation of Al3+ into Ce-Zr solid solutions may enhance the oxygen release reaction to form larger
Ce3+. In the present work, for the first time, we describe the preparation and characterization
of Ce0.5Zr0.3Al0.2O1.9 solid solutions with high surface area via a facile solvothermal route. The further
experiment results show that the introduction of aluminum ion enhances the thermal
stability and OSC even after calcination at a very strict condition of 1,000°C for
20 h. The OSC of CeO2, Ce0.5Zr0.5O2, and the composites which consisted of different aluminum amounts were also prepared
via the same method and compared.

Methods

All chemicals used were of analytical grade and were purchased from Kanto Chemical
Co. Inc., Tokyo, Japan (purity 99.999%). The chemicals were used without further purification.

Catalysts preparation

The stoichiometric amounts of (NH4)2Ce(NO3)6 (6 mmol), ZrO(NO3)2 (3.6 mmol), and Al(NO3)3·9H2O (2.4 mmol) were dissolved in 60 ml of distilled water. NH4OH solution was slowly dropped into the above mixed solution, and the pH value was
maintained at 9. The yellow mixed solution was introduced in a 100-ml Teflon®-lined
autoclave (SAN-AI Science, Co. Ltd, Nagoya, Japan), which was maintained at 200°C
for 24 h, then cooled to room temperature naturally. The obtained products were washed
with distilled water three times and dried in air at 100°C for 12 h to form the as-prepared
fresh samples. Finally, the fresh samples were calcined at 1,000°C for 20 h in air
atmosphere to evaluate the thermal stability. The same synthesis route was employed
for the preparation of the CeO2 and Ce0.5Zr0.5O2.

OSC analysis

The OSC of the samples calcined at 1,000°C for 20 h was determined by thermogravimetric-differential
thermal analysis (TG-DTA; Rigaku TAS-200, Rigaku Corporation, Tokyo, Japan) at 600°C.
Before the measurements, the samples were held in flowing air at 600°C for 30 min
to remove residual water and other volatile gases. The mixed gas of CO-N2 (100 cm3 min−1) and air (100 cm3 min−1) was flowed alternately at 600°C. Finally, OSC was analyzed after getting the TGA
profile.

Results and discussion

All products of (a) CeO2, (b) Ce0.5Zr0.5O2, and (c) Ce0.5Zr0.3Al0.2O1.9 consisted of a single phase of fluorite structure (Figure 1 (a) to (c)). All the diffraction patterns exhibited broad peaks, suggesting that
the fresh samples were nanocrystalline materials. The calcined samples had a slight
shift in diffraction peaks when compared to the pure CeO2 XRD pattern, indicating the formation of corresponding solid solutions. The calculated
lattice parameters of the calcined samples of Ce0.5Zr0.5O2 (a = 0.5384 nm) and Ce0.5Zr0.3Al0.2O1.9 (a = 0.5299 nm) are smaller than that of CeO2 (a = 0.5413 nm). The shrinkage of lattice cells may be due to the substitution of the
smaller cation radius of Zr4+ (0.084 nm) and Al3+ (0.0059 nm) with Ce4+ (0.097 nm). No phase separation was noticed even at such high calcination temperatures
at 1,000°C for 20 h, except the increase of particle size (Figure 1 (a') to (c')). The crystal sizes of the fresh CeO2, Ce0.5Zr0.5O2, and Ce0.5Zr0.3Al0.2O1.9 calculated by Scherer's formula were 9, 5, and 3 nm, while those of the calcined
CeO2, Ce0.5Zr0.5O2, and Ce0.5Zr0.3Al0.2O1.9 were 35, 10, and 8 nm, respectively.

The morphology and size of the fresh and calcined samples (1,000°C for 20 h) were
observed by TEM as shown in Figure 2. For the fresh samples, the particles seem to be partly dispersed and formed small
agglomerates (Figure 2 (a) to (c)), and the single particle exhibited a spherical-like morphology with the
diameters of 9 to 12 nm, 5 to 8 nm, and 3 to 5 nm for CeO2, Ce0.5Zr0.5O2, and Ce0.5Zr0.3Al0.2O1.9, respectively, which are in agreement with the crystallite size calculated from Scherer's
formula. The particle size increased after calcination at 1,000°C for 20 h because
of aggregation, and the particle sizes were found to increase to 90 to 100 nm, 50
to 55 nm, and 30 to 35 nm for the CeO2, Ce0.5Zr0.5O2, and Ce0.5Zr0.3Al0.2O1.9 samples as shown in Figure 2 (a') to (c'), respectively.

BET nitrogen adsorption-desorption analysis was undertaken to measure the specific
surface area of all samples. As a result, the fresh sample of Ce0.5Zr0.3Al0.2O1.9 showed a much higher surface area (232 m2 g−1) than those of CeO2 (119 m2 g−1) and Ce0.5Zr0.5O2 (168 m2 g−1, Figure 3 (a) to (c)). After calcinations at 1,000°C for 20 h in air, the specific surface
areas of CeO2 (3 m2 g−1) and Ce0.5Zr0.5O2 (8 m2 g−1) decreased to less than 10 m2 g−1, but the sample of Ce0.5Zr0.3Al0.2O1.9 exhibited a relatively higher BET specific surface area of 18 m2 g−1 (Figure 3 (a') to (c')).

The OSC values of the calcined samples were determined at 600°C with a continuous
flow of CO-N2 gas and air alternately. Figure 4 shows the typical TG profiles of the CeO2, Ce0.5Zr0.5O2, and Ce0.5Zr0.3Al0.2O1.9 samples. The TG profile shows the oxygen release/storage performance of the CeO2, Ce0.5Zr0.5O2, and Ce0.5Zr0.3Al0.2O1.9 samples at 600°C with time. As a result, Ce0.5Zr0.3Al0.2O1.9 exhibited a higher OSC of 427 μmol-O g−1, when compared to those of the CeO2 (25 μmol-O g−1) and Ce0.5Zr0.5O2 (350 μmol-O g−1) samples (Table 1). It is accepted that the OSC is dependent on the specific surface area; it is obvious
that Ce0.5Zr0.3Al0.2O1.9 exhibited the highest specific surface area and highest OSC values even after calcination
at such high temperature as 1,000°C for 20 h. In order to examine OSC performance
stability, oxygen release/storage cycle measurement was tested, and Ce0.5Zr0.3Al0.2O1.9 retained the same OSC even after 22 cycles (Figure 5). The result indicates that Ce0.5Zr0.3Al0.2O1.9 has good OSC performance stability.

The amount of incorporated aluminum was also controlled to test its effect on the
OSC of the calcined sample as shown in Figure 6 and Table 2. As a result, Ce0.5Zr0.3Al0.2O1.9 exhibited the highest OSC of 427 μmol-O g−1 (Table 1), when compared to those of the Ce0.5Zr0.4Al0.1O1.95 (378 μmol-O g−1), Ce0.5Zr0.2Al0.3O1.85 (389 μmol-O g−1), and Ce0.5Zr0.1Al0.4O1.8 (261 μmol-O g−1) samples (Table 2), Therefore, in Ce0.5Zr0.5-xAlxOy (0.1 <x < 0.5, x is the amount of incorporated aluminum), the most appropriate amount of incorporated
aluminum might be around x = 0.2.

Conclusions

Ce0.5Zr0.3Al0.2O1.9 solid solutions with high surface area were successfully synthesized via a facile
solvothermal method. The structures of the fresh samples and calcined samples were
characterized by X-ray diffraction. The lattice parameters of the Ce0.5Zr0.3Al0.2O1.9 solid solution are smaller than those of CeO2 and Ce0.5Zr0.5O2, suggesting the incorporation of the Al3+ into Ce-Zr solid solutions. The fresh particles showed spherical-like morphology
with a diameter of 3 to 5 nm determined by TEM. The Ce0.5Zr0.3Al0.2O1.9 solid solutions exhibited a remarkably higher oxygen storage capacity than those
of the CeO2 and Ce0.5Zr0.5O2 samples prepared via the same method, even after calcination at 1,000°C for 20 h,
indicating the improvement of the OSC and thermal stability due to the incorporation
of aluminum. An appropriate amount of incorporated aluminum is also suggested.

Competing interests

The authors declare that they have no competing interests.

Authors’ contributions

QD participated in the design of the study, carried out the total experiments, and
performed the result analysis as well as drafted the manuscript. SY participated in
the design of the study, gave the theoretical and experimental guidance, and made
the corrections of manuscript. CG mainly helped in the experiments and measurements.
TS gave the theoretical and experimental guidance and helped to amend the manuscript.
All authors read and approved the final manuscript.

Authors’ information

QD, SY, CG, and TS are an assistant professor, an associate professor, a Ph.D. candidate,
and a full professor, respectively, at the Institute of Multidisciplinary Research
for Advanced Materials, Tohoku University.

Acknowledgments

This work was supported by the Rare Metal Substitute Materials Development Project
of New Energy and Industrial Technology Development Organization (NEDO), Japan and
the Management Expenses Grants for National Universities Corporations from the Ministry
of Education, Culture, Sports and Science for Technology of Japan (MEXT).